IGF-I is a small polypeptide hormone that stimulates the growth of all types of cells. Because IGF-I has a broad spectrum of action and stimulates balanced tissue growth it has been implicated in the development of several important human cancers and also in atherosclerosis. IGF-I acts primarily on anchorage dependent cells that are contained in these tissues. These cells also possess a class of receptors termed integrin receptors which are responsible for their attachment to extracellular matrix molecules. In order for cells to divide normally, in response to extracellular stimuli the cell has to sense that its integrin receptors are bound to extracellular matrix molecules. Therefore manipulation of ligand occupancy of integrin receptors can alter processes that are important in disease development such as cell division and migration.
Our studies have determined that IGF-I stimulates endothelial and smooth muscle cell division. They have further determined that these cells utilize the αVβ3 integrin receptor to communicate to the cell nucleus that they are adhered adequately to extracellular matrix in order to divide. The abundance of one specific integrin (the αVβ3 integrin) is relatively restricted in human tissues and it is expressed primarily in growing cells and particularly in cells involved in the maintenance of the vasculature such as smooth muscle and endothelial cells. Our studies have shown that occupancy of this integrin receptor with its naturally occurring ligands such as osteopontin, vitronectin and thrombospondin is required for these cells to respond to IGF-I with increased DNA synthesis and cell migration. Blocking ligand occupancy of this integrin with disintegrin antagonists results in inhibition of cell growth and migration. Our studies have shown that this cooperative interaction between αVβ3 and the IGF-I receptor is mediated by regulating the translocation of two specific signaling molecules. These molecules are 1) a protein tyrosine phosphatase termed SHP-2 and 2) a signaling protein termed Shc. Under normal circumstances SHP-2 is localized in the cytoskeleton and cytosolic compartments of the cell. Following ligand occupancy of αVβ3 the cytoplasmic domain of the β3 integrin undergoes tyrosine phosphorylation. SHP-2 is transferred to the cell membrane by binding to proteins that bind to the phosphorylated tyrosine residues in β3. This transfer is necessary in order to localize SHP-2 to the membrane where it recruits other important signaling molecules such as Shc. SHP-2 colocalization with Shc and/or dephosphorylation of signaling molecules within the IGF-I signaling pathway is required for their activation and for subsequent transmission of signals from the IGF-I receptor to nucleus. Activation of the two major intracellular signaling pathways that are required for IGF-I activation (e.g. the PI-3 kinase and MAP kinase pathways) can be inhibited by inhibiting either SHP-2 or Shc transfer to the membrane. The site of localization of SHP-2 and Shc is a membrane protein termed SHPS-1. SHPS-1 is phosphorylated in response to IGF-I. This phosphorylation is required for SHP-2 and for Shc transfer. Shc is phosphorylated after transfer to SHPS-1. Blocking αVβ3 ligand occupancy blocks both SHP-2 and Shc transfer thus inhibiting IGF-I stimulated cell growth.
Although methods have been described previously for inhibiting ligand occupancy of the αVβ3 integrin, they all utilize a technology that inhibits binding to a specific binding site on the αVβ3 heterodimer that binds to the arginine, glycine, asparagine (RGD) sequence within the ECM ligands. Binding αVβ3 antagonists to this site is associated with drug toxicity and side effects. Accordingly there is a need for new ways to inhibit, or activate, IGF-1 actions that do not utilize the αVβ3 binding site that binds to the RGD sequence.